The invention relates to chitin/chitosan adsorbents that can be used in the nuclear industry and in nuclear power stations for removing radioactive contaminants from aqueous solutions, in particular elements such as cesium, uranium, plutonium americium, curium etc. They can also be used for cleaning wastes and drinking water of toxic heavy metals, such as lead, mercury, cadmium, bismuth, chromium etc.
In the nuclear industry and in the operation of nuclear power stations, a large amount of radioactive effluent is generated. This effluent represents a high risk to the health of people and to the environment, and its storage is both expensive and unreliable. To reduce the risk from radioactive effluents, they are converted into a solid state. Of the many possible methods, the adsorption method has considerable advantages, one of the main ones being that a considerable reduction in the volume is possible. The efficiency of the method depends on the quality of the adsorbent. It has been found that chitin adsorbents have many useful properties.
Chitin is a natural aminosugar-containing polysaccharide of (1.4)-linked 2-deoxy-2-acetamido-xcex2-D-glucose (N-acetyl-D-glucosamine). Chitosan is a deacetylated derivative of chitin. The high chemical and radioactive stability of chitin and chitiosan makes it possible to use these polymers for the extraction of radionuclides from aqueous solutions. The particular properties of the adsorption method have been investigated for many different radionuclides (Tsezos, 1980; Volesky, 1981; Muzzarelli, 1986; Muzzarelli et al. 1986; 1989; Erschov et al., 1992, 1993; Jansson-Charrier et al., 1994; Gorovoj, Kosyakov, 1994, 1996). Many of these works are concerned with the investigation of the potential for the extraction of uranium.
The sorption properties of chitin and its derivatives have proved sufficiently good for the extraction of uranium from aqueous solutions both in anionic and cationic form (Andreev et al., 1962). An effective method for the extraction of uranium from seawater was found in the use of chitin phosphate and chitosan phosphate, which result in a very high extraction rate (Sakaguchi, Taskashi et al., 1979, a, b, 1981). With this method, 1 g, of chitosan phosphate binds 2.6 mg of uranium if its content in seawater is 2.8 mg. An extraction rate of more than 90% is therefore achieved.
Crosslinked chitosan shows an increase in the adsorption of metals that attains 95%, this rise presumably being based on the amino group content (Kim, Choi, 1985 a). Chitosan showed here a better result than crosslinked chitosan (Kim, Choi, 1985 b). The adsorption of uranium by chitosan, which depends on the pH value, the size of the sorption particles and the presence of other metals, was investigated (Jansson-Charrier et al., 1994). The maximum adsorption was achieved with a pH value of 5, namely up to 400 mg/l. A drop in the size of the sorption particles showed a positive effect on the kinetics of the sorption process, which points to the limiting factor of diffusion. The adsorption of uranium fell when carbonates and phosphates were present.
The kinetics (speed) of uranium adsorption by chitosan is slow: a considerable improvement is however achieved when the chitosan is modified to chitosan phosphate or dithiocarbamate. In this case, the adsorption takes place in the first 15 minutes. The highest adsorption indexes for uranium were achieved with the derivative N-[2(1,2-dehydrooxyethl)tetrahydrofuryl] chitosan, which has a capacity of up to 800 mg/g (Muzzarelli et al., 1984).
The bioadsorption of uranium also occurs very easily in the chitin of the cell walls of fungi, in particular rhizopus arrhizus (Tsezos, Volesky, 1982 a; Tsezos, 1983). It was demonstrated that with a pH value of 2 the state of balance is reached after 3 to 4 hours, and during this time 1 mg of uranium is adsorbed per 1 g of cell wall. With a pH value of 4, up to 180 g of uranium are adsorbed in the cell walls, while pure chitin absorbs only 6 mg(g of uranium with this pH value, even though the cell walls of the fungi are not 100% chitin.
The use of chitin for cleaning of water contaminated with radioactive substances, in particular plutonium, can present a solution for the problem of neutralizing the waste generated during the extraction, enrichment and use of nuclear fuels. The contact between chitin particles and radioactive liquid is achieved by a multiple-stage mixing and settling process, counterflow process and passage through towers. The cleaned water and the contaminated chitin are separated with the aid of gravity separation or filtration. Using this method, more than 80% of the plutonium can be extracted from aqueous solutions with a pH value of between 5 and 10 (Silver, 1978).
It is known that chitin and the chitin-containing material chisit-03 effectively extracts plutonium (IV), americium (III) and curium (III). The distribution coefficients rise in the order: Pu(IV) less than Cm(III) less than Am(III). The adsorption of Pu(V) and Pu(VI) by chisite-03 is considerably better than the adsorption by chitin, and is comparable with that of Pu(IV) (Ershov et al., 1992 a).
The high radiation stability of chitin and chitosan has made it possible to perform the investigation of these materials for the concentration of effluents from nuclear fuels (radioactive isotopes of cesium, zirconium, hafnium and other elements) from water circulating in reactor cooling systems. Cesium is, like other alkaline metals, not adsorbed by chitin and chitosan. Of the other nuclear fission elements, tests were conducted on rare-earth elements such as cerium, europium, thulium and terbium (Muzzarelli et al., 1972; Lopez de Alba et al., 1988 a). The extraction rate (in %) of these metals by chitin was low, between 3 and 9%. Chitosan had better results, but these too were not high: between 30 and 45%. The extraction of ruthenium from waste water from the nuclear power station at Marcoulle also showed poor results. Up to 2 and 4% of this metal were adsorbed in chitin, and up to 60% in chitosan (Muzzarelli, 1970, 1977). Chitin/chitosan-containing material of fungi (higher basidiomycetes) showed better adsorption results for uranium, plutonium, americium, curium and heavy metals (Gorovoj, Kosyakov, 1994, 1996; Kosyakov et al., 1997).
The use of a chitin adsorbent for extraction of radionuclides from aqueous solutions was promising in this respect. There are however unsolved problems in this field which do not permit the industrial use of chitin adsorbents. The main problem is that it is not possible to remove cesium isotopes from radioactive effluents. Cesium is a principal component in the radioactive contamination of effluents both in nuclear technology and in nuclear power stations. Another drawback of the known chitin adsorbent is the low efficiency of extraction of plutonium ions and other radionuclides from effluent. These drawbacks practically prevent any solution to the cleaning of radioactive effluents.
The problem underlying the present invention was therefore to provide an adsorbent for radionuclides, such as cesium, transuraniums etc. with which the problems occurring to date can be solved.
This problem is solved by the present invention.
The subject matter of the invention is an adsorbent.
A further subject matter is a process for the manufacture of the adsorbent in accordance with the invention.
A further subject matter is also a process for the cleaning of salt-containing radioactive effluents.
A further subject matter is also the use of an adsorbent in accordance with the invention for the cleaning of radioactive effluents, in particular from the nuclear industry and from nuclear power stations.
In accordance with the present invention, new properties are imparted to the chitin-containing material obtained from fungi (higher basidiomycetes) in order to adsorb radioactive cesium from water and concentrated saline solutions. In accordance with the invention, it is also possible to improve considerably the adsorption properties of a chitin-containing material in respect of such radionuclides, for example uranium, plutonium, americium and curium.
This is achieved in accordance with the invention by incorporating ferrocyanides of the transition metals in a chitin-containing material, for example copper ferrocyanide, allowing it to be converted into a microcrystalline insoluble state. At the expense of their reactive groups, they become a new adsorbent capable of combining with radioactive cesium. The ferrocyanide microcrystals loosen the adsorbent matrix and increase their surface, and improve the penetration of a solution to the reactive centers of chitin.
A chitin-containing material for manufacture of an adsorbent from natural or cultivated fungi (higher basidiomycetes) is described in the Russian patent 2073015 (Gorovoj, Kosyakov, 1997).
To impart adsorption properties to a chitin-containing material in respect of radioactive cesium, it is proposed to incorporate additional reactive groups which are effective for combining with this chemical element. To increase the adsorption properties in respect of plutonium or other radionuclides, it is proposed to increase the possibility for a contact of heavy metal ions with reactive centers of the chitin by loosening the structure of the chitin-containing material.
In accordance with the invention, ferrocyanides of transition metals are proposed as the preferable modifying substances. Ferrocyanides comprise normal salts (e.g. Me2Fe(CN)6) and mixed salts [M4xe2x88x922xMexFe(CN)6], in which M is a monovalent metal cation and Me a bivalent cation of a transition metal. Ferrocyanides of transition metals show a high selectivity in respect of ions of heavy alkali metals, including the radioactive isotopes of cesium. Copper ferrocyanide has several advantages when compared with ferrocyanides of other metals: it has the lowest solubility (approx. 10xe2x88x925 mol/l) and the highest distribution coefficients for cesium (Kd=5.105).
Insoluble ferrocyanides of transition metals can be obtained by direct precipitation as the result of reduction of suitable soluble ferrocyanides in accordance with the following reaction equation: 
where Red=reduction means, Ox=oxidation means Chitin-containing materials have certain reduction properties.
If K+ and Fe(CN)64xe2x88x92 ions are present, the reaction can lead to the formation of various compounds: 
For the reaction to lead to (1), the 1:2 ratio of Cu2+to Fe(CN)64xe2x88x92 must be retained, and the reaction performed in an ammonia solution with an ammonia concentration of 0.1 to 2 mol/l. Experiments have confirmed that the preferred concentration of ammonia in a solution is in the range from 0.3 to 0.7 mol/l.
The chitin-containing material described in Russian patent 2073015 represents a chitin-glucane-melanin complex of natural biopolymers from a cell wall of fungi (higher basidiomycetes). The material has a fine-fibered structure. Chitin is present in the form of microfibrils with a diameter of 150 to 250 xc3x85 and a length from up to 1 to up to 2 xcexcm. The chitin microfibrils are inside the amorphous glucane-melanin matrix, thus assuring that a microfibril spatial network structure is maintained. At the same time, the glucane-melanin complex prevents direct contact by chitin microfibrils with the solution, which reduces the effectiveness of the extraction of radionuclides such as uranium, plutonium, americium, curium etc. and heavy metals.
This drawback is avoided in accordance with the invention, and the contact of a solution with chitin microfibrils is improved by loosening the glucane-melanin matrix by means of introducing ferrocyanide microcrystals into the matrix mass. To achieve this, the material is impregnated with soluble salts containing Cu2+ and Fe(CN)63xe2x88x92, and reduction is then performed. To achieve the conversion, it is necessary to add ammonia to the suspended material. Inside and on the surface of the glucane-melanin matrix, microcrystals of copper ferrocyanide are formed that loosen the matrix.
The invention is now demonstrated with the following examples, without being restricted to these: